Modern society has come to depend in an essential manner on valuable information stored in archives, hopefully securely. The physical security level of the traditional medium, paper, is however very low. Wear and tear, water, fire and chemical agents present in the air can destroy paper on a short time scale. With the birth of the computer age, two new means have been introduced for archival storage. One is the magnetic recording, the second one is the optical disk.
The security level of magnetic recording is lower than that of paper: valuable information can be erased in little time by mistake, mischief, malice, or unfortunate decisions. In addition a variety of physical causes can destroy magnetically recorded information. These include mechanical failures of the read-out equipment (e.g. head crashes), spontaneous magnetic domain reversals, heat, fire, and stray magnetic fields, including those produced by high-power electromagnetic pulses from lightning or hostile man-made devices.
The second widespread archival medium is the optical disk, i.e. the ubiquitous compact disk called “CD” for music and CD-ROM for data, and the increasingly popular dense video disk or “DVD”. The optical disk's physical security level is moderately high. Provided the disk is not exposed to excessive heat or light (e.g. direct sunlight), and provided the surfaces are protected from excessive scratching, the stored information can be guaranteed for a lifetime, in the 10- to 30-year range. That still leaves fire and excessive heat as potential agents undermining the physical security of current optical disks.
The need for ever larger data storage capacities has led the optical disk industry towards increasing both the surface density of information carrying pits and the number of layers hosting these information pits. The industry has progressed from the compact disk storing 650 MB on one 86-cm2 layer (or 7.5 MB/cm2), to the latest four-layer DVD which can store 17 GB over the same 86-cm2 area of access to information. For the four-layer DVD this represents an access density of 0.2 GB/cm2.
Despite the impressive storage capacity of the latest DVD, the construction of large Petabyte-range archives, would require tens to hundreds of thousands of disks, and its operation would necessitate the use of cumbersome juke-box-type retrieval mechanisms for rapid remote access. Such mechanical handling of large numbers of disks would diminish the security and reliability of the archival memory.
In the on-going effort towards increasing the access density of information, the optical disk industry has gone from one layer in the CD, to two and four layers in the DVD, and towards even more layers in research carried out in a number of laboratories.